Skype doesn't show notification for a specific contact

If you don't receive skype-notifications when one specific person writes you,
that's mean it is Disabled :) I don't know how, but suddenly I found the same bug in my Skype.
A bug - because I didn't disable it.

How to fix it ?!

Buuuut... I haven't "Notification settings" at all :)
Instead I found another solution - you can enable notifications for any contact just by sending /alertson into chat with him.

Here some more useful commands from official site. For more information, please, look there.

/add [Skype Name] : Adds a contact to the chat.
For instance: /add alex_cooper will add that member to the chat.
/alertson [text] : Allows you to specify what needs to appear in a chat for you to be notified.
For example, /alertson London will only alert you when the word "London" appears in the chat.
/alertsoff : Disable message notifications.
/get role : Details your role in the chat.
/get uri : Creates a URL link that other people can use to join the group chat.
/golive : Starts a group call with other participants of the chat.
/info : Details number of people in chat and maximum number available.
/me [text] : Your name will appear followed by any text you write.
For instance, "/me working from home" will cause the phrase "working from home" to appear next to your name in the chat. You can use this to send a message about your activities or status.
/topic [text] : Changes the chat topic.
/undoedit : Undo the last edit of your message.
/whois [Skype Name] : Provides details about a chat member such as current role.